Field of the Invention
The present invention relates to a positive electrode active material for non-aqueous electrolyte secondary batteries, production process therefor, and non-aqueous electrolyte secondary battery. More particularly, the invention relates to a positive electrode active material that uses a nickel-cobalt-manganese composite oxide particle as a raw material, a production process therefor, and a non-aqueous electrolyte secondary battery that uses the positive electrode active material as a positive electrode material. It is to be noted that the present application claims priority based on the Japanese Patent Application No. 2014-157102 filed on Jul. 31, 2014 in Japan.
Description of Related Art
In recent years, along with the popularization of mobile electronic devices such as cellular phones and lap-top personal computers, the development of small and light non-aqueous electrolyte secondary batteries with a high energy density has been desired strongly. In addition, as batteries for electric vehicles including hybrid vehicles, the development of high-output secondary batteries has been desired strongly.
There are lithium ion secondary batteries as secondary batteries that meet these requirements. The lithium ion secondary batteries are each composed of a negative electrode and a positive electrode, and an electrolytic solution or the like, and materials capable of desorbing and inserting lithium are used as active materials for the negative electrode and the positive electrode.
Lithium ion secondary batteries have been actively researched and developed now, and above all, lithium ion secondary batteries that use layered or spinel-type lithium metal composite oxides for positive electrode materials achieve high voltages on the order of 4 V, which have been thus progressively put into practical use as batteries with a high energy density.
As positive electrode materials for lithium ion secondary batteries, currently, composite oxides have been proposed, such as lithium-cobalt composite oxides (LiCoO2) which are relatively easily synthesized, lithium-nickel composite oxides (LiNiO2) and lithium-nickel-cobalt-manganese composite oxides (LiNi1/3Co1/3Mn1/3O2) which use more inexpensive nickel than cobalt, lithium-manganese composite oxides (LiMn2O4) which use manganese, and lithium-nickel-manganese composite oxides (LiNi0.5Mn0.5O2).
Among the positive electrode materials, in recent years, lithium-nickel-cobalt-manganese composite oxides (LiNi0.33Co0.33Mn0.33O2) have been attracting attention, which are excellent in thermal stability and high in capacity. The lithium-nickel-cobalt-manganese composite oxides (LiNi0.33Co0.33Mn0.33O2) have layered compounds as with the lithium-cobalt composite oxides and the lithium-nickel composite oxides, and contain, at the transition metal site, nickel, cobalt, and manganese in proportions basically at a composition ratio of 1:1:1.
Now, as conditions of allowing a positive electrode to achieve excellent performance (high cycle characteristics, low resistance, high output), the positive electrode material is required to be composed of particles with a uniform and appropriate particle size, and further required to be produced so as to be dispersed uniformly in the electrode.
This is due to the fact that the use of a material that is large in particle size and small in specific surface area fails to ensure a sufficient area for a reaction with an electrolytic solution, thereby increasing the reaction resistance, and thus failing to achieve a high-output battery. In addition, the use of a material that has a broad particle size distribution results in unevenness of the voltage applied to particles in the electrode, and thus, repeated charge and discharge selectively degrade microparticles, thereby decreasing the capacity. Furthermore, when the dispersed state is not uniform, the flow of current is biased, thereby causing problems such as a decrease in battery capacity and an increase in reaction resistance.
As a positive electrode active material that is highly uniform in particle size and small in particle size, Patent Document 1 discloses a positive electrode active material of a lithium-nickel-cobalt-manganese composite oxide composed of a hexagonal lithium-containing composite oxide that has a layered structure, where the positive electrode active material is 2 to 8 μm in average particle size, and 0.60 or less in [(d90−d10)/average particle size] as an index indicating the breadth of a particle size distribution. This positive electrode active material is supposedly capable of achieving great battery output characteristics and high capacities, but the dispersibility in electrodes is not considered.
Causes of biased dispersibility in the electrode include, for example, the influence of the alkalinity of the positive electrode active material. It is known that when the alkalinity of the positive electrode active material is high, the alkali attacks and polymerizes a binder (PVDF) commonly used in the adjustment of a positive electrode paste, thereby causing the positive electrode paste to gelate, and thus failing to apply the paste evenly. The reduction in the alkalinity of the positive electrode active material is conceivable as a process for preventing the situation.
As a positive electrode active material with alkalinity reduced, the positive electrode active material with the Mn/Ni ratio controlled therein has been proposed. For example, Patent Document 2 discloses a positive electrode active material of a lithium-nickel-cobalt-manganese composite oxide composed of a hexagonal lithium-containing composite oxide that has a layered structure, where the positive electrode active material is 3 to 12 μm in average particle size, 0.60 or less in [(d90−d10)/average particle size] as an index indicating the breadth of a particle size distribution, and pH=10.6 to 11.5 in alkalinity, and a process for producing the positive electrode active material.
Patent Document 1: JP 2011-116580 A
Patent Document 2: JP 2012-256435 A
However, in accordance with the production process described in Patent Document 2, the Mn/Ni ratio in an aqueous solution for use at the stage of the formation of outer peripheral parts of secondary particles is made higher than that in an aqueous solution at the stage of the formation of inner parts thereof in the production of nickel-manganese composite hydroxide particles as a precursor for the positive electrode active material. Further, because the aqueous solutions are switched in the process of the crystallization reaction, productivity is decreased in the production on an industrial scale, and uniformity between the particles is less likely to be achieved therein. Moreover, the increased ratio of Mn/Ni in the outer peripheries of the secondary particles of the lithium-manganese-nickel composite oxide has the problem of difficulty in achieving high outputs.
As mentioned above, there is no example of a report that the alkalinity of the positive electrode active material is reduced while the output is increased satisfactorily. In addition, currently, on an industrial scale, lithium-nickel-cobalt-manganese composite oxides that can sufficiently improve the performance of non-aqueous electrolyte secondary batteries have not been developed, but the development has been desired.